The present invention relates to a metal line of a semiconductor device and a method for forming the same, and more particularly, to a metal line of a semiconductor device that exhibits improved physical and performance characteristics which is results in improving the reliability of a resultant semiconductor device and a method for forming the same.
Generally, in a semiconductor device, metal lines are formed to electrically connect elements or lines to each other. Contact plugs are formed to connect lower metal lines and upper metal lines to each other. As the high integration of a semiconductor device continues to proceed, the design rules necessarily result in gradually increasing the aspect ratios of a contact hole in which a contact plug is to be formed. As a result, the process for forming metal lines and contact plugs become more difficult because these diminutive metal lines and contact plugs must also maintain their physical and performance characteristics such as assuring that unwanted impurities do not diffuse past certain barriers.
Aluminum and tungsten have been mainly used as conductive materials in the metal lines of a semiconductor device because they exhibit relatively good electrical conductivity properties and because they do not tend towards adversely affecting the performance of the resultant semiconductor device due to unwanted diffusion away from these metal lines. Recently, research has been made in the hopes of using copper as a next-generation material for a metal line because copper exhibits superior electrical conductivity and low resistance as compared to aluminum and tungsten. Copper (Cu) can therefore solve or at least aid in minimizing the problems associated with RC (resistance-capacitance) signal delay in the semiconductor device having a high level of integration and high operating speed.
It is known that copper diffuses very fast through semiconductor substrates and through insulation layers. Further the diffused copper is known to act as a deep-level impurity in the semiconductor substrate made of silicon and can induce leakage currents. Therefore, it is necessary to form a diffusion barrier at an interface between a copper layer acting as a metal line and the surrounding insulation layer. Generally, the diffusion barrier made of TaN/Ta layers is able to restrain copper from diffusing beyond the confines of the metal line.
Hereinbelow, a conventional method for forming a metal line of a semiconductor device will be briefly described.
After forming an insulation layer on a semiconductor substrate designated to have a metal line forming region, TaN/Ta layers serving as a diffusion barrier and a copper seed layer are sequentially formed on the surface of the insulation layer. The Ta layer functions to increase the adhesion force between the diffusion barrier and the copper seed layer. Then, after forming a copper layer on the copper seed layer, by using CMPing (chemically and mechanically polishing) on the copper layer, a copper metal line is formed.
However, in the conventional art described above, it is necessary to form a thick Ta layer so as to increase the adhesion force between the diffusion barrier and the copper seed layer. Due to this fact, in the conventional art described above, the thickness of the diffusion barrier increases, and the thickness of the copper layer necessarily decreases because of the geometric constraints of the diminutive structure, whereby the characteristics of the copper metal line deteriorates.
Also, in the conventional art described above, in order to reduce the agglomeration of copper, the copper seed layer should be formed through PVD (physical vapor deposition) at a low temperature. Due to this fact, in the conventional art described above, the step coverage of the copper seed layer becomes poor, which leads to the formation of unwanted overhangs. As a consequence, the entrance to the metal line forming region is likely to be clogged which in turn results in forming voids along and within the metal line.
These problems are getting worse as the design rule of a semiconductor device gradually decreases and as a result the characteristics and the reliability of the resultant semiconductor devices can deteriorate.